GHC doesn't allow Coercion between partly-saturated type constructors

If I define an "opposite category" newtype Op cat a b = Op (cat b a) then representationally it forms an involution: applying Op twice gives the same representation as not applying it at all

$ ... -ignore-dot-ghci
GHCi, version 8.3.20170920: http://www.haskell.org/ghc/  :? for help
Prelude> import Data.Coerce
Prelude Data.Coerce> import Data.Type.Coercion 
Prelude Data.Coerce Data.Type.Coercion> newtype Op cat a b = Op (cat b a)
Prelude Data.Coerce Data.Type.Coercion> :t coerce :: Op (Op cat) a b -> cat a b
coerce :: Op (Op cat) a b -> cat a b :: Op (Op cat) a b -> cat a b
Prelude Data.Coerce Data.Type.Coercion> :t Coercion :: Coercion (Op (Op cat) a b) (cat a b)
Coercion :: Coercion (Op (Op cat) a b) (cat a b)
  :: Coercion (Op (Op cat) a b) (cat a b)

But these don't work:

Prelude Data.Coerce Data.Type.Coercion> :t Coercion :: Coercion (Op (Op cat) a) (cat a)

<interactive>:1:1: error:
    • Couldn't match representation of type ‘Op (Op cat1) a1’
                               with that of ‘cat1 a1’
        arising from a use of ‘Coercion’
    • In the expression: Coercion :: Coercion (Op (Op cat) a) (cat a)
Prelude Data.Coerce Data.Type.Coercion> :t Coercion :: Coercion (Op (Op cat) cat

<interactive>:1:38: error:
    parse error (possibly incorrect indentation or mismatched brackets)
Prelude Data.Coerce Data.Type.Coercion> :t Coercion :: Coercion (Op (Op cat)) cat

<interactive>:1:1: error:
    • Couldn't match representation of type ‘cat1’
                               with that of ‘Op (Op cat1)’
        arising from a use of ‘Coercion’
      ‘cat1’ is a rigid type variable bound by
        an expression type signature:
          forall (cat1 :: * -> * -> *). Coercion (Op (Op cat1)) cat1
        at <interactive>:1:13-38
    • In the expression: Coercion :: Coercion (Op (Op cat)) cat

I'm wondering if this is intentional

Trac metadata

Trac field	Value
Version	8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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